US aid worker Peter Kassig has been beheaded by the Islamic State group.
The 26-year-old, who was captured last year, was pictured in a graphic video released by IS in which a black-clad militant claimed to have beheaded Mr Kassig, who was captured last year.
The White House tonight said a review of the video confirmed Mr Kassig's death.
That announcement came as President Barack Obama flew back to Washington from Hawaii after a trip last week to the Asia Pacific region.
Mr Kassig served in an Army special operations unit in Iraq and after he was medically discharged, he formed an aid organisation in Turkey to help Syrian refugees.
The Indianapolis man delivered food and medical supplies, and provided care to wounded Syrian civilians before he was seized in eastern Syria last year.
In the video, the militant known as Jihadi John, stands over the body.
The video, which was posted on websites used by the group in the past, appeared to be the latest in a series of blood-soaked messages warning of further brutality if the US and its allies do not abandon an air campaign in Iraq and Syria.
"This is Peter Edward Kassig, a US citizen, of your country; Peter who fought against the Muslims in Iraq, while serving as a soldier," the militant says near the end of the 16-minute video. He speaks in an audible British accent despite his voice being distorted to make it more difficult to identify him.
The video identifies the militant's location as Dabiq, a small town in the northern Syrian province of Aleppo, near the Turkish border.
The video also shows what appears to be the mass beheading of several Syrian soldiers captured by the group. The militants warn that US soldiers will meet a similar fate.
"We say to you, (US president) Obama... you claim to have withdrawn from Iraq four years ago," the militant said. "Here you are: you have not withdrawn. Rather, you hid some of your forces behind your proxies," he said, apparently referring to Western-backed Syrian rebels, Kurdish fighters and the Iraqi military.
"Here we are, burying the first American crusader in Dabiq, eagerly waiting for the remainder of your armies to arrive."
Mr Kassig, a former US Army Ranger, was providing medical aid to Syrians fleeing the civil war when he was captured inside Syria on October 1, 2013. His friends say he converted to Islam in captivity and took the first name Abdul-Rahman.
Previous videos have shown the beheading of two American journalists and two British aid workers. The latest video did not show the person identified as Mr Kassig being beheaded. And unlike previous videos, it did not show other Western captives or directly threaten to behead anyone else.
The group also holds British photojournalist John Cantlie, who has been shown in several videos delivering long statements in English on the group's behalf, perhaps under duress.
Mr Kassig's parents, Ed and Paula Kassig of Indianapolis, released a statement through a family spokeswoman.
"We are aware of the news reports being circulated about our treasured son and are waiting for confirmation from the government as to the authenticity of these reports." They said they would have no additional comment and requested privacy.
Mr Kassig's friends and family say he understood the risks involved in working in the region, but that he felt called to help.
Burhan Agha, a 26-year-old Syrian, used to work with Mr Kassig in the northern Lebanese city of Tripoli, delivering aid to Syrian refugees before he moved his operations to southern Turkey. Speaking from Switzerland, where he is seeking asylum, Mr Agha described his friend's killing as senseless.
"If I could apologise to each American, one by one, I would," Mr Agha said. "Because Peter died in Syria, while he was helping the Syrian people. And those who killed him claimed to have done it in the name of Islam. I am a Muslim, and from Syria, and he is considered a part of the Syrian revolution."
The video emerged just minutes after Mr Obama left Australia for the US after the G20 economic summit.
Mr Kassig formed the aid organisation Special Emergency Response and Assistance, or SERA, in Turkey to provide aid and assistance to Syrian refugees. He began delivering food and medical supplies to Syrian refugee camps in 2012 and is also a trained medical assistant who provided trauma care to wounded Syrian civilians. His friends say he helped train 150 civilians in providing medical aid.
The Islamic State group has beheaded and shot dead hundreds of captives - mainly Syrian and Iraqi soldiers - during its sweep across the two countries, and has celebrated its mass killings in a series of videos.
The US began launching air strikes in Iraq and Syria earlier this year in a bid to halt the group's rapid advance and eventually degrade and destroy it.